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Cam lobe decel ramp - valve life?

General engine tech -- Drag Racing to Circle Track

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David Redszus
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Re: Cam lobe decel ramp - valve life?

Post by David Redszus »

No answers to my question about recommended max seating velocity limit values.
The factors affecting valve seating wear are not related to valve velocity.

They are a function of valve acceleration which will impart a force to the valve greater than
the resisting spring.

This sometimes occurs at the opening ramp but mostly on the flank of the cam.

When the lifter misses the ramp, due to excessive lash, it will impact the flank resulting
in an acceleration spike.

Even if the lifter stays planted on the ramp, it may still see excessive acceleration (force) due to
cam design, excessive rpm, heavy valve train or low valve spring seat force. Or all of the above.

Valve bounce is most often a result of valve float during closing when a lifter loses contact with
the cam lobe, sails over the nose and crashes on the closing ramp. Now the valve might bounce as
much as 1/3 of the total lift.

Carefully measure the ramp height and duration; lash clearance should be such as to locate closing
contact at mid-ramp. If set too tight, the valve may not close properly resulting in overheated valves.

If set too loose, the lifter will crash into the cam flank.

Now where is a Spintron when you really need one?
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Re: Cam lobe decel ramp - valve life?

Post by hoffman900 »

Nut124 wrote: Tue Sep 08, 2020 11:11 pm
SchmidtMotorWorks wrote: Tue Sep 08, 2020 9:05 pm Does the lobe transition to the base circle look smooth to the eye?
Yes. Looks perfect.

I reshimmed #1 intake valve to 0.003 clearance and will try to get the degree wheel on the crank to measure the valve closing in very small increments. This is just so I can measure the full cam lobe profile to assess seating velocity at different clearances.

Need to shim the degree wheel 3/8" off the face of the pulley in order not to remove water pump.

No answers to my question about recommended max seating velocity limit values.
It really is a "just depends" type answer. Most cam designers have their own thoughts but it also depends on the type of valvetrain and what that particular Fiat can get away with.
-Bob
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Re: Cam lobe decel ramp - valve life?

Post by CamKing »

Nut124 wrote: Tue Sep 08, 2020 3:28 pm
What kind of max velocity limits are being recommended for valve seating?
You want the velocity at the lash point to be under .0008" per degree. If it's over .001", I would worry.
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Re: Cam lobe decel ramp - valve life?

Post by Nut124 »

CamKing wrote: Wed Sep 09, 2020 10:31 am
Nut124 wrote: Tue Sep 08, 2020 3:28 pm
What kind of max velocity limits are being recommended for valve seating?
You want the velocity at the lash point to be under .0008" per degree. If it's over .001", I would worry.
Mike, good info. just what I needed. Thanks! Will take me a day or two to get numbers... Busy making vaccine.
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Re: Cam lobe decel ramp - valve life?

Post by Stan Weiss »

CamKing wrote: Wed Sep 09, 2020 10:31 am
Nut124 wrote: Tue Sep 08, 2020 3:28 pm
What kind of max velocity limits are being recommended for valve seating?
You want the velocity at the lash point to be under .0008" per degree. If it's over .001", I would worry.
Mike,
Do you ever look at valve seating velocity in fps @ shift point RPM?

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Re: Cam lobe decel ramp - valve life?

Post by CamKing »

Stan Weiss wrote: Wed Sep 09, 2020 11:49 am Mike,
Do you ever look at valve seating velocity in fps @ shift point RPM?
Yes, I calculate the velocity at the lash point in fpm, based on lobe velocity at lash point, rocker ratio, and max RPM.
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Re: Cam lobe decel ramp - valve life?

Post by Nut124 »

CamKing wrote: Wed Sep 09, 2020 10:31 am
Nut124 wrote: Tue Sep 08, 2020 3:28 pm
What kind of max velocity limits are being recommended for valve seating?
You want the velocity at the lash point to be under .0008" per degree. If it's over .001", I would worry.
Mike, here is the data. It kind of confirms what I'm hearing. In this test, I have the #1 valve lash adjusted to 0.005 and measuring valve travel on the bucket:

If I adjusted lash to 0.010, the seating velocity would be around 1.1 - 1.5 thou/deg.

What do you think?

Image

Image
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Re: Cam lobe decel ramp - valve life?

Post by Nut124 »

Nut124 wrote: Wed Sep 09, 2020 11:26 am
CamKing wrote: Wed Sep 09, 2020 10:31 am
Nut124 wrote: Tue Sep 08, 2020 3:28 pm
What kind of max velocity limits are being recommended for valve seating?
You want the velocity at the lash point to be under .0008" per degree. If it's over .001", I would worry.
Mike, good info. just what I needed. Thanks! Will take me a day or two to get numbers... Busy making vaccine.
Mike, just to confirm: The 0.0008/deg maximum is based on crank degrees, not cam degrees, right?
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Re: Cam lobe decel ramp - valve life?

Post by CamKing »

Nut124 wrote: Thu Sep 10, 2020 9:26 am Mike, just to confirm: The 0.0008/deg maximum is based on crank degrees, not cam degrees, right?
No, it's cam degrees.
sorry I didn't clarify that.
.0008" per cam degree
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Re: Cam lobe decel ramp - valve life?

Post by Nut124 »

CamKing wrote: Thu Sep 10, 2020 11:01 am
Nut124 wrote: Thu Sep 10, 2020 9:26 am Mike, just to confirm: The 0.0008/deg maximum is based on crank degrees, not cam degrees, right?
No, it's cam degrees.
sorry I didn't clarify that.
.0008" per cam degree
Mike, thanks for the clarification. With cam degrees, this cam looks even worse, seating velocities in the 3-4 thou/cam deg vs 0.8/deg max recommendation.

What do you think?

Image

Image
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Re: Cam lobe decel ramp - valve life?

Post by CamKing »

Yes, that looks like it will be hard on the valvetrain.
To me, it really looks like the lobe was originally designed for hydraulic buckets.
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Re: Cam lobe decel ramp - valve life?

Post by PRH »

Or...... looking at it another way........ an effective valve lash of “0”?
Somewhat handy with a die grinder.
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Re: Cam lobe decel ramp - valve life?

Post by williamsmotowerx »

What does the opening side look like? Speculating cam could have been ground backwards?
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Re: Cam lobe decel ramp - valve life?

Post by Nut124 »

williamsmotowerx wrote: Fri Sep 11, 2020 12:05 am What does the opening side look like? Speculating cam could have been ground backwards?
It's the same.
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Re: Cam lobe decel ramp - valve life?

Post by Nut124 »

I appreciate the help, thanks. Particularly Mike - Thank you.

I removed the "bad" cam and installed the old faithful "Alquati" 40-80 cam I had ran for 20 years. I would like a bit more cam, but suppliers are out. The old cam will run "quiet" and may produce OK power when timed correctly. I ran it for 20 years with incorrect timing, about 5-8 deg retarded, reducing overlap.

When I put in the old cam with the same shims, there was no valve clearance. Had to remove and install 0.030 less shim to get in the ballpark. This shows that the old cam has way less lift.

Below are a few pics:

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